I'm not entirely convinced that this is entirely some sort of widespread bad behavior. Many non-profit boards conduct research on salaries and essentially size their organization and pay something akin to a market rate for the given size and scope.
However, even a small percentage of bad actors finding a way to inflate their salaries will, as a side effect, inflate salaries across the board because it influences the process that sets the salaries for the honest organizations.
It's a fun problem.
I suspect abuse is more prevalent at the low end, among nonprofits that don’t do much.
I stand by the point of my original post: People often underestimate how much the leaders of nonprofits pay themselves. These are figures you can look up and quiz your friends to test the hypothesis, if they’re into that sort of thing. For a good time include some nonprofit hospitals.